The presence of hepatitis B core antibody (HB.sub.c Ab) in biological fluids is a diagnostic marker of current or previous hepatitis B disease. Biological material such as, e.g, serum or plasma from such individuals also may contain hepatitis B surface antigen (HB.sub.s Ag) or Dane particles in levels capable of transmitting disease although below levels detectable in presently available assays. A difficulty with tests heretofore known for HB.sub.c Ab, e.g., RIA or IAHA, is their short shelf life of only a few weeks and in the case of the IAHA assay the requirement for specialized red blood cells which are difficult to obtain.